EFFECTS OF CENTRAL METAL-ION (MG, ZN) AND SOLVENT ON SINGLET EXCITED-STATE ENERGY-FLOW IN PORPHYRIN-BASED NANOSTRUCTURES

Zinc porphyrins have been widely used as surrogates for chlorophyll (w hich contains magnesium) in photosynthetic model systems and molecular photonic devices. In order to compare the photodynamic behaviour of M g- and Zn-porphyrins, dimeric and starshaped pentameric arrays compris ed of free-base (Fb) and Mg- or Zn-porphyrins with intervening diaryle thyne linkers have been prepared. A modular building block approach is used to couple ethynyl- or iodo-substituted porphyrins in defined met allation states (Fb, Mg or Zn) via a Pd-catalysed reaction in 2-6 h. T he resulting arrays an purified in 45-80% overall yields by combinatio ns of size exclusion chromatography and adsorption chromatography (gre ater than or equal to 95% purity). High solubility of the arrays in or ganic solvents Facilitates chemical and spectroscopic characterization . The star-shaped Mg(4)Fb- and Zn(4)Fb-pentamers, where the Fb-porphyr in is at the core of the array, have pairwise interactions similar to those of dimeric MgFb- and ZnFb-arrays. The arrays have been investiga ted by static and time-resolved absorption and fluorescence spectrosco py, as well as resonance Raman spectroscopy. The major findings includ e the following. (1) The rate of singlet excited-state energy transfer from the Mg-porphyrin to the Fb-porphyrin [(31 ps)(-1)] is comparable to that from the Zn-porphyrin to the Fb-porphyrin [(26 ps)(-1)] in th e dimeric arrays. Qualitatively similar results are obtained for the s tar-shaped pentamers. The similar rates of energy transfer for the Mg- and Zn-containing arrays are attributed to the fact that the electron ic coupling between the metalloporphyrin and Fb-porphyrin is approxima tely the same for Mg-vs. Zn-containing arrays. (2) The quantum yield o f energy transfer is slightly higher in the Mg-arrays (99.7%) than in the Zn-arrays (99.0%) due to the longer inherent lifetime of Mg-porphy rins (10 ns) compared with Zn-porphyrins (2.5 ns). (3) The rate of ene rgy transfer and the magnitude of the electronic coupling are essentia lly independent of the solvent polarity and the coordination geometry of the metalloporphyrin (four- or five-coordinate for Zn-porphyrins, f ive- or six-coordinate for Mg-porphyrins). (4) Polar solvents diminish the fluorescence yield and lifetime of the excited Fb-porphyrin in ar rays containing either Mg- or Zn-porphprins. These effects are attribu ted to charge-transfer quenching of the Fb-porphyrin by the adjacent m etalloporphyrin rather than to changes in electronic coupling. The mag nitude of the diminution is greater for the Mg-containing arrays, whic h is due to the greater driving force for charge separation. (5) The Z n-containing arrays are quite robust while the Mg-containing arrays ar e slightly labile toward demetallation and photooxidation. Taken toget her, these results indicate that porphyrin-based nanostructures having high energy-transfer efficiencies can be constructed from either Mg- or Zn-porphyrins. However, Mg-containing arrays may be superior in sit uations where a succession of energy-transfer steps occurs (due to a s lightly higher yield per step) or where charge transfer is a desirable feature. On the other hand, Zn-porphyrins are better suited when it i s desirable to avoid charge transfer quenching reactions. Accordingly, the merits of constructing a device from Mg-vs. Zn-containing porphyr ins will be determined by the interplay of all of the above factors.